Why don’t we auto-save for users instead of having them save manually?

With the advent of Apple moving to auto-saving in its newest release Lion, should everyone start adopting the convention of auto-saving?

At first it is definitely awkward and the user can feel like they have less control, but if widely adopted this could make everyone's lives easier. Of course we should still offer a "Save as" or "Create a copy" type option to allow them to manually save a copy.

Are there any large technical/performance hurdles regarding auto-saving and if not, should we all start adopting the auto-save technique?

If we (Windows and other platforms) do not adopt this new convention, it will certainly create an even larger gap between Windows and Mac users. People who have only used Macs or grow up using them from here on out will not be used to saving. People who use Windows will be driven crazy by Macs (some not all) because they will constantly want to save.

Inspiration taken from an answer on Save icon, is the floppy disk icon dead?